After months of negotiating behind closed doors, the Senate issued a 295-page reform proposal that goes much further than most had expected in opening Mexico’s oil and gas fields to international investment.

“It is groundbreaking,” said Gabriel Salinas, an attorney with Mayer Brown, who has followed the reform debates closely. “It provides what companies will need to go down there and develop Mexico’s resources. I think the bill has everything that Mexico needs to have a competitive energy sector, like that of Brazil or Colombia.”

The proposed constitutional amendment would break the decades-old monopoly that Mexico’s national company, Petroleos Mexicanos, or Pemex, has enjoyed, requiring it for the first time to compete potentially with other companies for projects.

“It’s the biggest change you could imagine,” said Steve Otillar, a Houston-based partner specializing in the emerging market energy sector for Akin Gump, an international law firm. “The government and Pemex have historically been joined at the hip — they are a designated monopoly in the constitution, and that is one of the key things that is being changed.”

An earlier proposal by President Enrique Peña Nieto would have allowed companies to receive cash payments, called profit sharing contracts, in exchange for participation. But this proposal goes much further, allowing companies the option to share in the actual oil production, or to contract independently of Pemex.

“It seems that the notion of profit sharing has been dropped,” said George Baker, publisher of Mexico Energy Intelligence. “What they are now talking about is that the government is looking for the flexibility to have contracting terms and conditions that make sense, given the geology and the nature of the risk and the technology required.”

The proposal comes after multiple rounds of failed energy reform proposals. Many credit the stagnant economy for propelling the majority party, the Institutional Revolutionary Party, or PRI, into working with the conservative National Action Party, or PAN, to craft the proposal.

Many are speculating that the new rules, if passed, will transform Mexico’s economy.

“I can’t understate how important this is — this is the beginning of the reformation of the entire energy sector in Mexico,” Otillar said. “It will help Mexico enjoy the energy renaissance taking place in the United States and modernize its economy on the back of energy reform.”

Both Congress and state legislatures are expected to vote on the proposal by Sunday, when Congress begins a winter recess that lasts until February.

Because the PRI and PAN parties have the needed two-thirds majority votes to amend the constitution, the proposal is expected to pass.

While the proposal clearly states that the Mexican government retains ownership of the oil until it is produced, opponents to the proposal say it will open the door to privatization of Mexico’s energy resources. Since its nationalization of the oil sector in 1938, Mexico has defined much of its national identity on control of its natural resources, and the demonstrations organized by the leftist Democratic Revolution Party over the past several months reflect this debate.

The proposal also falls short of expressly permitting international oil and gas companies to book expected oil production as reserves in their financial statements, which had been specifically banned under a 1958 law. Reserves, which reflect a company’s future expected production, are one of the most important Wall Street measurements for assessing values. The ability to book reserves is critical for considering investment in Mexico.

The real test of the legislation will be in the hoped-for investments it attracts from international operators, who have broadly hinted that if reform happens, they will come.

“The industry as a whole has always sought to define a role in Mexico,” Lee Tillman, CEO of Houston-based Marathon Oil Corp. said in August. “It is a tremendous hydrocarbon province. Should we see reform, just like any hydrocarbon resource, there will be ready suitors there to take a hard look at Mexico as an opportunity.”

Deputies vote during a session of the lower house of the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. Mexico will end 75 years of government control of its vast oil reserves after Congress approved the nations most significant economic reform since the North American Free Trade Agreement. less

Deputies vote during a session of the lower house of the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. Mexico will end 75 years of government control of its vast oil reserves after ... more

Photo: Susana Gonzalez / Bloomberg

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Manlio Fabio Beltrones (right), a deputy with the Institutional Revolutionary (PRI), shakes hands with Silvano Aureoles of the Party of the Democratic Revolution (PRD), after the final vote on a proposal to reform the oil industry at the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. Mexico will end 75 years of government control of its vast oil reserves after Congress approved the nations most significant economic reform since the North American Free Trade Agreement. less

Manlio Fabio Beltrones (right), a deputy with the Institutional Revolutionary (PRI), shakes hands with Silvano Aureoles of the Party of the Democratic Revolution (PRD), after the final vote on a proposal to ... more

Photo: Susana Gonzalez / Bloomberg

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Manlio Fabio Beltrones (right), a deputy with the Institutional Revolutionary (PRI); Silvano Aureoles (center) of the Party of the Democratic Revolution (PRD); and Luis Alberto Villareal of the National Action Party (PAN), speak at the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. Mexico will end 75 years of government control of its vast oil reserves after Congress approved the nations most significant economic reform since the North American Free Trade Agreement. less

Manlio Fabio Beltrones (right), a deputy with the Institutional Revolutionary (PRI); Silvano Aureoles (center) of the Party of the Democratic Revolution (PRD); and Luis Alberto Villareal of the National Action ... more

Photo: Susana Gonzalez / Bloomberg

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An opposition lawmaker shouts while holding a protest banner as dozens of leftist lawmakers take over the lower house trying to block discussion of the energy reform bill in Mexico City, Wednesday, Dec. 11, 2013. less

An opposition lawmaker shouts while holding a protest banner as dozens of leftist lawmakers take over the lower house trying to block discussion of the energy reform bill in Mexico City, Wednesday, Dec. 11, ... more

Photo: Marco Ugarte / Associated Press

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A man wears a helmet with a miniature replica of the deep-water drilling platform and a message that reads in Spanish; "Pemex is not for sale," during protests against a proposed energy reform bill, near the Senate building in Mexico City, Thursday, Dec. 5, 2013. less

A man wears a helmet with a miniature replica of the deep-water drilling platform and a message that reads in Spanish; "Pemex is not for sale," during protests against a proposed energy reform bill, near the ... more

Photo: Eduardo Verdugo / Associated Press

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A banner with a slogan against proposed oil industry reforms (top) hangs above a banner with a slogan in support of the reform, at the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. Mexico's lower house passed an energy bill that ends Petroleos Mexicanos' 75-year oil monopoly in a bid to attract foreign investment and boost growth. less

A banner with a slogan against proposed oil industry reforms (top) hangs above a banner with a slogan in support of the reform, at the Mexican Congress in Mexico City, Mexico, on Thursday, Dec. 12, 2013. ... more

Photo: Susana Gonzalez / Bloomberg

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Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Photo: Marco Ugarte / Associated Press

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Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Photo: Marco Ugarte / Associated Press

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Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Opposition lawmakers occupy the podium at the National Congress in protest against the newly approved energy reform bill in Mexico City, Wednesday, Dec. 11, 2013.

Photo: Marco Ugarte / Associated Press

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Mexico's Democratic Revolution party (PRD) lawmakers make a barricade to seal the entrances to the chamber of deputies on December 11, 2013 in Mexico City. Leftist lawmakers locked themselves inside Mexico's chamber of deputies Wednesday to prevent the lower house from debating a controversial oil reform bill that had passed the Senate. less

Mexico's Democratic Revolution party (PRD) lawmakers make a barricade to seal the entrances to the chamber of deputies on December 11, 2013 in Mexico City. Leftist lawmakers locked themselves inside Mexico's ... more

Photo: LIBRADO BAEZ / AFP/Getty Images

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A National Regeneration Movement (MORENA) supporter holds a sign against privatizing Petroleos Mexicanos, or Pemex, during a demonstration outside the Mexican Senate in Mexico City, Mexico, on Wednesday, Dec. 4, 2013. less

A National Regeneration Movement (MORENA) supporter holds a sign against privatizing Petroleos Mexicanos, or Pemex, during a demonstration outside the Mexican Senate in Mexico City, Mexico, on Wednesday, Dec. ... more

Photo: Susana Gonzalez / Bloomberg

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A demonstrator stands with signs that read "No To The Theft Of All Time" in front of federal police guarding the Mexican Senate in Mexico City, Mexico, on Friday, Dec. 6, 2013.

A demonstrator stands with signs that read "No To The Theft Of All Time" in front of federal police guarding the Mexican Senate in Mexico City, Mexico, on Friday, Dec. 6, 2013.

Photo: Susana Gonzalez / Bloomberg

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Demonstrators shout slogans against political reforms in front of riot police guarding outside at the National Congress in protest against the newly approved energy reform bill in Mexico City, Thursday, Dec. 12, 2013. less

Demonstrators shout slogans against political reforms in front of riot police guarding outside at the National Congress in protest against the newly approved energy reform bill in Mexico City, Thursday, Dec. ... more

Photo: Marco Ugarte / Associated Press

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A man dressed as a wrestler holds up a banner that reads in Spanish; "Pemex is not for sale," during a protest against a proposed energy reform bill near the Senate building in Mexico City, Thursday, Dec. 5, 2013. less

A man dressed as a wrestler holds up a banner that reads in Spanish; "Pemex is not for sale," during a protest against a proposed energy reform bill near the Senate building in Mexico City, Thursday, Dec. 5, ... more

Photo: Eduardo Verdugo / Associated Press

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A protester wears a hat with a small oil well and a Mexican flag as he stands outside of the Senate building in Mexico City, Sunday, Dec. 8, 2013.

A protester wears a hat with a small oil well and a Mexican flag as he stands outside of the Senate building in Mexico City, Sunday, Dec. 8, 2013.

Photo: Marco Ugarte / Associated Press

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A woman walks next to a fence in front of the Mexican Senate on December 4 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the privatization of the state-owned oil company Petroleos Mexicanos (Pemex). Lopez Obrador, who never recognized being defeated by Pena Nieto in the 2012 presidential election after claiming fraud, rejects Pena Nieto's plans to overhaul the tax system and open the country's state-controlled oil industry to foreign investors. less

A woman walks next to a fence in front of the Mexican Senate on December 4 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the privatization of ... more

Photo: RONALDO SCHEMIDT / AFP/Getty Images

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Mexican policemen stand guard next to a fence in front of the Mexican Senate on December 5 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the privatization of the state-owned oil company Petroleos Mexicanos (Pemex). Lopez Obrador, who never recognized being defeated by Pena Nieto in the 2012 presidential elections, rejects Pena Nieto's plans to overhaul the tax system and open the country's state-controlled oil industry to foreign investors. less

Mexican policemen stand guard next to a fence in front of the Mexican Senate on December 5 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the ... more

Photo: RONALDO SCHEMIDT / AFP/Getty Images

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Mexican police officers stand guard next to a fence in front of the Mexican Senate on December 5, 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the privatization of the state-owned oil company Petroleos Mexicanos (Pemex). Lopez Obrador, who never recognized being defeated by Pena Nieto in the 2012 presidential election, rejects Government's plans to overhaul the tax system and open the country's state-controlled oil industry to foreign investors. less

Mexican police officers stand guard next to a fence in front of the Mexican Senate on December 5, 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against ... more

Photo: RONALDO SCHEMIDT / AFP/Getty Images

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A man rides his bicycle in front of Mexican police officers in the Mexican Senate on December 5, 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against the privatization of the state-owned oil company Petroleos Mexicanos (Pemex). Lopez Obrador, who never recognized being defeated by Pena Nieto in the 2012 presidential elections, rejects Government's plans to overhaul the tax system and open the country's state-controlled oil industry to foreign investors. less

A man rides his bicycle in front of Mexican police officers in the Mexican Senate on December 5, 2013, in Mexico City, as supporters of Mexico's leftist leader Andres Manuel Lopez Obrador hold a rally against ... more

Photo: RONALDO SCHEMIDT / AFP/Getty Images

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A supporter of Andres Manuel Lopez Obrador's left wing movement MORENA hits with a spoon a metalic barrier of the Mexican Congress during a protest against an energy proposal reform in Mexico City on December 11, 2013. less

A supporter of Andres Manuel Lopez Obrador's left wing movement MORENA hits with a spoon a metalic barrier of the Mexican Congress during a protest against an energy proposal reform in Mexico City on December ... more

Photo: YURI CORTEZ / AFP/Getty Images

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Supporters of former presidential candidate Andres Manuel Lopez Obrador cheer as they listen to Lopez Obrador during an act to protest against the government's proposed energy reforms that would allow private companies to explore the country's oil and gas reserves, in Mexico City, Sunday Sept. 8, 2013. The proposed reform requires constitutional changes that strike at the heart of one of Mexico's proudest moments: President Lazaro Cardenas' nationalization of the oil company in 1938. (AP Photo/Marco Ugarte) less

Supporters of former presidential candidate Andres Manuel Lopez Obrador cheer as they listen to Lopez Obrador during an act to protest against the government's proposed energy reforms that would allow private ... more

Photo: Marco Ugarte / Associated Press

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Supporters of former presidential candidate Andres Manuel Lopez Obrador cheer as they listen to Lopez Obrador during an act to protest against the governments proposed energy reforms that would allow private companies to explore the country's oil and gas reserves, in Mexico City, Sunday Sept. 8, 2013. The proposed reform requires constitutional changes that strike at the heart of one of Mexico's proudest moments: President Lazaro Cardenas' nationalization of the oil company in 1938. (AP Photo/Marco Ugarte) less

Supporters of former presidential candidate Andres Manuel Lopez Obrador cheer as they listen to Lopez Obrador during an act to protest against the governments proposed energy reforms that would allow private ... more

Photo: Marco Ugarte / Associated Press

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Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, waves after delivering a speech against the energetic reform proposed by Mexican President Enrique Pena Nieto, during a rally along Juarez Avenue in Mexico City, on September 8, 2013. less

Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, waves after delivering a speech against the energetic reform proposed by Mexican President Enrique Pena ... more

Photo: YURI CORTEZ / AFP/Getty Images

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Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, gestures as he delivers a speech against the energetic reform proposed by Mexican President Enrique Pena Nieto, during a rally along Juarez Avenue in Mexico City, on September 8, 2013. less

Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, gestures as he delivers a speech against the energetic reform proposed by Mexican President Enrique Pena ... more

Photo: YURI CORTEZ / AFP/Getty Images

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Supporters of Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, attend a rally along Juarez Avenue against the energetic reform proposed by Mexican President Enrique Pena Nieto, in Mexico City, on September 8, 2013. less

Supporters of Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, attend a rally along Juarez Avenue against the energetic reform proposed by Mexican President ... more

Photo: YURI CORTEZ / AFP/Getty Images

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Supporters of Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, shout slogans against the energetic reform proposed by Mexican President Enrique Pena Nieto, during a rally along Juarez Avenue in Mexico City, on September 8, 2013. less

Supporters of Mexican former presidential candidate and leader of the MORENA movement, Andres Manuel Lopez Obrador, shout slogans against the energetic reform proposed by Mexican President Enrique Pena Nieto, ... more

Photo: YURI CORTEZ / AFP/Getty Images

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A man holds a banner that reads in Spanish, "Pemex is not for sale," at a political rally for presidential candidate Andres Manuel Lopez Obrador in the Zocalo in Mexico City. President Enrique Pena Nietos proposal to revamp energy policy and modernize Pemex, expected to be unveiled in August 2013, is igniting an enormous political fight, not just for the oil industry but for the entire country. (AP Photo/Alexandre Meneghini, File) less

A man holds a banner that reads in Spanish, "Pemex is not for sale," at a political rally for presidential candidate Andres Manuel Lopez Obrador in the Zocalo in Mexico City. President Enrique Pena Nietos ... more

Photo: Alexandre Meneghini / Associated Press

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Enrique Pena Nieto speaks during an interview at the Los Pinos presidential residence in Mexico City in December 2012.

Enrique Pena Nieto speaks during an interview at the Los Pinos presidential residence in Mexico City in December 2012.

Photo: Dario Lopez-Mills / Associated Press

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A masked demonstrator holds up a Mexican flag during a march to protest against what they claim is an alleged government plan to privatize PEMEX, the state-owned oil company, and also to mark the one year anniversary of the election of Mexican President Enrique Pena Nieto in Mexico City, Monday, July 1, 2013. less

A masked demonstrator holds up a Mexican flag during a march to protest against what they claim is an alleged government plan to privatize PEMEX, the state-owned oil company, and also to mark the one year ... more

Photo: Ivan Pierre Aguirre / Associated Press

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A truck drives past a refinery in Salamanca, Mexico, 170 miles (280 kilometers) northwest of Mexico City on Saturday, Jan. 19, 2002. President Enrique Pena Nieto's proposal to revamp energy policy and modernize the state-owned Petroleos Mexicanos (Pemex), expected to be unveiled in August 2013, is igniting an enormous political fight, not just for the oil industry but for the entire country. less

A truck drives past a refinery in Salamanca, Mexico, 170 miles (280 kilometers) northwest of Mexico City on Saturday, Jan. 19, 2002. President Enrique Pena Nieto's proposal to revamp energy policy and ... more

Photo: Marco Ugarte / Associated Press

Image 31 of 34

A Mexican police officer tries to make his way through the crowd during a march to protest against what they claim is an alleged government plan to privatize PEMEX, the state-owned oil company, and also to mark the one year anniversary of the election of Mexican President Enrique Pena Nieto in Mexico City, Monday, July 1, 2013. less

A Mexican police officer tries to make his way through the crowd during a march to protest against what they claim is an alleged government plan to privatize PEMEX, the state-owned oil company, and also to ... more

Photo: Ivan Pierre Aguirre / Associated Press

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This Jan. 13, 2005 aerial file photo shows Los Pajaritos petrochemical complex that belongs to Mexico's state-owned oil company Petroleos Mexicanos (Pemex) in Coatzacoalcos in the state of Veracruz, Mexico. President Enrique Pena Nietos proposal to revamp energy policy and modernize Pemex, expected to be unveiled in August 2013, is igniting an enormous political fight, not just for the oil industry but for the entire country. less

This Jan. 13, 2005 aerial file photo shows Los Pajaritos petrochemical complex that belongs to Mexico's state-owned oil company Petroleos Mexicanos (Pemex) in Coatzacoalcos in the state of Veracruz, Mexico. ... more

Photo: Dario Lopez-mills / Associated Press

Image 33 of 34

Mexico's President Enrique Pena Nieto attends a ceremony marking the anniversary of the federal police at the federal police intelligence center in Mexico City, Friday, July 12, 2013.

Mexico's President Enrique Pena Nieto attends a ceremony marking the anniversary of the federal police at the federal police intelligence center in Mexico City, Friday, July 12, 2013.